"Sudden change" refers to a quick and unexpected transformation or shift in a situation or circumstance. It implies that something happens without warning or preparation, which can bring both surprises and challenges. Full definition
There have been instances of sudden change in my conceptual viewpoint, and some may be evident in this overview. (millyristvedt.ca)
A somewhat sudden change of heart for the titular character who once refused to be called a hero implants a level of soul no one could have expected. (comicbook.com)
Since cats don't do well with sudden changes in routine we tried hard to keep things simple. (floridawildvethospital.com)